1. The Field of the Invention
This invention relates generally to radio transceivers operating in full duplex mode. More particularly, the invention relates to dynamically controlling the bias level in a receiver portion of a transceiver to prevent compression by the transmitter power.
2. Present State of the Art
Modern communication devices in order to facilitate the user's communication behavior, accommodate the simultaneous transmission of both spoken or data information and the reception of similar information. Such a simultaneous exchange in a user transceiver is known as full duplex communication. Users of communicating devices have become accustomed to such simultaneous transmit and receive capabilities in wired communicating devices such as standard telephones. To prevent the transmitting signal from contaminating a received signal, traditional telephone systems have employed isolated wires or differentiating propagation modes to facilitate the dedicated exchange of information. Likewise, in wireless or radio communication devices employing full duplex transceivers, the transmitter and receiver of the full duplex transceiver device employ separate communication frequencies to minimize interference between the simultaneous transmitting and receiving signals. Such a frequency isolation provides the necessary separation for the information once it is broadcast into the propagation medium such as the ether about the full duplex transceiver. However, full duplex transceivers utilize a single internal conduit through which both transmit and receive signals are routed to the full duplex transceiver's antenna.
Those skilled in the art of electromagnetic propagation appreciate that the signal levels of the transmit signal are many orders of magnitude larger within the transceiver than are the received signals as presented to the transceiver. For example, transmission signals prior to exiting the full duplex transceiver may exhibit signal levels on the order of 8 dBm to 28 dBm while the received signals at the transceiver are on the order of -113 dBm. It should be appreciated that such a discrepancy in power signal magnitudes may result in the received signal being wholly overwhelmed by the signal levels of the transmit signal. To prevent such a condition from occurring, full duplex transceivers have employed a duplexer which provides a barrier or isolation between the transmitter portion of the transceiver and the receiver portion of the transceiver. Such a duplexer largely filters the transmitter signal from overwhelming the received signal. While duplexers do not entirely remove the transmitter signal as seen at the receiver portion of the transceiver, at a minimum, the duplexer reduces the transmit signal level, as perceived at the receiver, to a signal level more similar to those of the received signal.
In a traditional receiver, the reduced transmit signal level and the received signal are thereafter processed through a low-noise amplifier to boost both signals to a usable level such that additional circuitry may extract the received signal from the contaminating affects of the transmit signal. To provide the aforementioned isolation between the transmit and receive signals, duplexers have taken on rather large form-factors in transceivers. Radio transceivers have likewise taken on various form factors such as vehicle-mounted transceivers commonly used by dispatch services, and modernly even more portable versions include handheld cellular telephones. As mentioned previously, because of the isolation requirements of a duplexer in minimizing the transmitter's transmit signal level as perceived by the receiver, duplexers are required to assume a substantial dimension. While such a substantial dimension may be tolerated by even handheld transceiver devices such as portable cellular telephones, when a transceiver device is reduced in size much beyond a handheld form factor, the duplexer dimension becomes a driving and dominant restriction upon the overall transceiver device form factor.
In order to accommodate smaller form-factor transceivers, the isolation capability of the duplexer must be compromised. However, by compromising the duplexer isolation capability, the large signal levels of the transmit signal contaminate the receive signal by overwhelming the dynamic range of the receiver amplifiers. That is to say, receiver amplifiers in order to accurately replicate the signal as received, must amplify such a signal to a larger and more usable signal level for further processing. Those skilled in the art, recognize that amplifiers are not wholly linear devices. That is to say, amplifiers have a linear region in which operation is more favorable and also have a non-linear region in which distortion may be injected into the received signal. When transmit signals are of a sufficient magnitude when presented to an amplifier such as a low-noise amplifier (LNA), the higher signal levels associated with the contaminating transmit signal drive the LNA into a non-linear region thereby distorting the received signal. Also, when driven into compression by the transmitter signal, the effective gain for the received signal is reduced and the received signal may not be amplified enough for proper receiver operation. To prevent the larger transmit signals from driving the LNA into such a compressive or distortive non-linear state, the bias level of the LNA must be increased in order to raise the linear and non-distortive region of operation of the LNA. Such an increase in the bias level taxes the overall power of the transceiver device. While transceiver devices having unlimited available power may tolerate the additional syphoning of system power to accommodate higher bias levels for the higher transmit signal levels, portable devices operating on resources such as battery power, cannot tolerate such an impact to the overall system performance.
Another functional detail of some modern full duplex applications is that full duplex transceivers operating in certain environments such as cellular environments interact with base stations at various locations. In such systems, in order to minimize interference with adjacent base stations, a base station may notify a transceiver of the signal quality as received at the base station of the signal as transmitted by the transceiver. When the base station determines the signal amplitude, and hence the transmit power level, to be in excess of that required for tolerable communication, the base station notifies the transceiver of a lower power level setting adequate for interaction. While a lower transmit power level may allow the bias level of the LNA to be reduced, the LNA must be able to accommodate the highest power level of the transmitter in order to prevent the LNA from going into a compression state.
Thus, what is needed is a method and system for tolerating the higher transmit power levels presented at the receiver front-end in a full duplex transceiver due to reduced isolation therebetween, without causing the receiver front-end to enter a distortion-creating non-linear region. What is yet further needed is a method and system for minimizing the bias level power consumption of a receiver front-end when the transmit power of the transceiver varies.